The invention relates to imaging, aiming and target illumination in optical code reading devices. Aspects of the invention are particularly useful in solid state, area image sensor based, handheld code readers which are positioned at variable orientations and distances with respect to a target code.
Optical codes are patterns made up of image areas having different light reflective or light emissive properties, which are typically assembled in accordance with a priori rules. The term xe2x80x9cbarcodexe2x80x9d is sometimes used to describe certain kinds of optical codes. The optical properties and patterns of optical codes are selected to distinguish them in appearance from the background environments in which they are used. Devices for identifying or extracting data from optical codes are sometimes referred to as xe2x80x9coptical code readersxe2x80x9d of which barcode scanners are one type. Optical code readers are used in both fixed or portable installations in many diverse environments such as in stores for check-out services, in manufacturing locations for work flow and inventory control and in transport vehicles for tracking package handling. The optical code can be used as a rapid, generalized means of data entry, for example, by reading a target barcode from a printed listing of many barcodes. In some uses, the optical code reader is connected to a portable data processing device or a data collection and transmission device. Frequently, the optical code reader includes a handheld sensor which is manually directed at a target code.
Most conventional optical scanning systems are designed to read one-dimensional barcode symbols. The barcode is a pattern of variable-width rectangular bars separated by fixed or variable width spaces. The bars and spaces have different light reflecting characteristics. One example of a one dimensional barcode is the UPC/EAN code used to identify, for example, product inventory. An example of a two-dimensional or stacked barcode is the PDF417 barcode. A description of PDF417 barcode and techniques for decoding it are disclosed in U.S. Pat. No. 5,635,697 to Shellhammer et al., and assigned to Symbol Technologies, Inc., which is incorporated herein by reference. Another conventional optical code is known as xe2x80x9cMaxiCodexe2x80x9d. It consists of a central finder pattern or bull""s eye center and a grid of hexagons surrounding the central finder. It should be noted that the aspects of the inventions disclosed in this patent application are applicable to optical code readers, in general, without regard to the particular type of optical codes which they are adapted to read. The invention described may also be applicable to some associated image recognition or analysis.
Most conventional scanning systems generate one or more beams of laser light which reflects off a barcode symbol and back to the scanning system. The system obtains a continuous analog waveform corresponding to the light reflected by the code along one or more scan lines of the system. The system then decodes the waveform to extract information from the barcode. A system of this general type is disclosed, for example, in U.S. Pat. No. 4,251,798, assigned to Symbol Technologies, Inc. A beam scanning system for detecting and decoding one and two dimensional barcodes is disclosed in U.S. Pat. No. 5,561,283, also assigned to Symbol Technologies, Inc.
Barcodes can also be read employing imaging devices. For example an image sensor may be employed which has a two dimensional array of cells or photo sensors which correspond to image elements or pixels in a field of view of the device. Such an image sensor may be a two dimensional or area charge coupled device (CCD) and associated circuits for producing electronic signals corresponding to a two-dimensional array of pixel information for a field of view.
Many scanners in use today employ a scanning laser beam. Some such systems are deployed in handheld units which may be manually pointed at the target. Often an individual scanner is a component of a much larger system including other scanners, computers, cabling, data terminals, etc. Such systems are frequently designed and constructed on the basis of mechanical and optical specifications for the scanning engine, sometimes called xe2x80x9cform factorsxe2x80x9d. One such form factor is the SE 1200 form factor employed by Symbol Technologies, Inc.
It is an object of the present invention to provide a compact imaging engine which can be substituted for conventional laser line scanning engines in currently designed and currently deployed optical code reader systems.
It is another object of the present invention to provide an imaging engine which can be substituted for SE1200 form factor scanning engines in currently designed and currently deployed optical code reading systems to increase the reliability, versatility and target working range of such systems.
It is known in the art to use a CCD photo detector and objective lens assembly in an optical code reader. In the past, such systems have employed complex objective lenses assemblies originally designed for use in relatively expensive video imaging systems. Such lens assemblies typically employ multiple, large diameter, aspheric lens elements. Use of aspheric lens elements and a CCD photo detector in a code reader is illustrated in U.S. Pat. No. 5,703,349. Aspheric lens systems are relatively costly and difficult to build. They also have a single sharp focus and a limited depth of field, which along with conventional aiming, illumination and signal processing and decoding algorithms, limits the versatility and working range of the system.
Applicants assignee, Symbol Technologies, Inc. has developed bi-stable high speed zone collection systems for barcode scanners. Systems which employed lens structures moveable into the input optical path of the scanner (drop-in optics) are disclosed in U.S. patent application Ser. Nos. 08/627,167 and 08/627,168 filed Apr. 3, 1996 to Li et al.
It is an object of the present invention to provide an easily constructed and inexpensive objective lens assembly for an imaging optical code reader.
It is another object of the present invention to provide an optical code reader which can be used to read codes at a wide range of distances.
It is another object of the present invention to provide an imaging optical code reader with selectable fields of view and working depths of view appropriate to the signal processing and decoding capabilities of the reader.
Various approaches have been proposed for aiming optical code readers. The simplest approach is to use the familiar red projected laser lines from a laser line scanner as an indication of the aim of the device. In passive, imaging systems, it has been proposed to project beams toward the target for aiming purposes. The device shown in U.S. Pat. No. 5,703,349 projects laser light from two targeting lens elements to form so called xe2x80x9chot spotsxe2x80x9d in the target area at an angle corresponding to the horizontal field of view of the system. An improved aiming system is disclosed in U.S. patent application Ser. No. 08/444,387 assigned to Symbol Technologies, Inc., the contents of which is incorporated herein by reference. This system employs a single laser diode and a diffractive optic element to produce multiple spots or beamlets for identifying corners or edges of a target area. This system uses different wavelength light for illumination and aiming, and appropriate optical filtering to eliminate the aiming pattern from the detected image.
It is an object of the present invention to provide an improved aiming system for an optical code reader.
It is another object of the present invention to provide a simply and inexpensively fabricated aiming system for an optical code imaging engine.
It is another object of the present invention to project an aiming pattern useable by the operator to aim the device and to provide data useful to the overall system.
It is another object of the present invention to provide an optical code reader which can use an aiming system to measure distances to a target.
It is another object of the present invention to provide an aiming system adaptable with aspects of the present invention, elsewhere disclosed herein, for providing various optical fields and focal distances in an optical imaging engine.
It is known to provide illumination in optical code readers employing image sensors as a supplement to ambient light. For example, U.S. Pat. No. 5,703,349 discloses an illumination module comprised of two lines of illuminating LEDs and lens cells. Such systems are adapted for use in single working range systems.
It is an object of the present invention to provide an illumination system for an optical code reader which is readily fabricated and provides sufficiently intense and uniform illumination for code reading.
It is another object of the present invention to provide an illumination system for an optical code reader which provides illumination adapted for different working ranges.
Some or all of the objects previously described may be achieved in a single optical code reading engine or system. With the addition of appropriate control circuitry and data processing software, a system may be constructed serving the object of producing a compact, inexpensively fabricated imaging engine which may be substituted for existing line scan engines. The engine may be adapted for use in many different environments, with various optical fields and focal distances, for reading various codes of different size. The system may also be used for image recognition or analysis, including acquisition of data concerning the target and its environment.
These and other objects and features of the invention will be apparent from this written description and drawings.
The present invention relates to methods and apparatus useful in optical code readers, especially imaging optical code readers. Techniques are disclosed which are applicable to the design of imaging engines, imaging lens systems, and aiming, illumination and signal processing devices associated with code readers of various types.
An imaging and aiming apparatus for an optical code reader may be based on an image sensor, including an array of photo sensor cells for producing electronic signals corresponding to a two-dimensional array of pixel information for a field of view. In preferred embodiments, the image sensor is a charge coupled-device (CCD), it being understood that other area image sensors may be used for the purpose such as CMOS, CMD (charge modulated device) or CID (charge injection device) sensors. A lens assembly is provided for focusing light incident on the image sensor. In preferred embodiments the lens assembly is switchable between at least two different fields of view, corresponding to at least two different focal distances. Advantageously, a control mechanism is provided to rapidly switch the lens assembly between fields of view and fixed focal distance lenses. In preferred embodiments, an aiming pattern generator projects at least two different aiming frames, each indicating a field of view of the lens assembly.
The above-described apparatus may constitute part of an imaging engine which also includes a power supply, decoding circuitry and video controller circuitry. In preferred embodiments, the imaging engine is less than two cubic inches in volume and is dimensioned to replace a moving laser beam scanning engine in a handheld barcode scanner, such as an SE1200 form factor scanning engine. Such an imaging engine may be designed to read a variety of types of optical codes including high and low density barcodes at a working range of between 1xc2xd and 18 inches, or more.
The aiming pattern generator of preferred embodiments of the present invention produces multiple aiming frames having different, partially overlapping, solid angle fields or dimensions corresponding to the different fields of view of the lens assembly. The aiming pattern may also include a centrally located marker or cross hair pattern. Each aiming frame may consist of four corner markers each comprising a plurality of illuminated spots, for example two multiple spot lines intersecting at an angle of 90xc2x0.
The aiming pattern generator may include at least one laser diode. A laser light beam from the diode impinges at least one diffractive optical element. In preferred embodiments, the diode beam is folded in order to align, in parallel, the principal optical axis of the aiming pattern generator with the principal axis of the lens assembly such that there is a small offset between the two, typically no more than 1xe2x80x3.
The image sensor may sense an image of all or a portion of the aiming pattern, particularly the center marker or cross-hair pattern. Image analysis circuitry or software may be provided to process pixel information from two images of the same target field of view, one with and one without an image of the centrally located marker. The distance of a target code may be determined based on the apparent location of the centrally located marker in the field of view. In furtherance of this approach, the aiming pattern generator may be periodically turned off during which time pixel information of the target field is obtained for decoding without the image of the centrally located marker.
The imaging systems of the present invention may further include frame grabbing circuitry for providing video signals for displaying images produced by the apparatus on a terminal monitor. In this case hardware modification of existing terminals may be avoided. Software or hardware switching may be provided for selectively turning off the aiming pattern projector when the apparatus is used to obtain pixel information for image display, for example when the device is used to produce an image of the human face, without projecting the laser aiming pattern on the face.
In preferred embodiments of the present invention the lens assembly includes first and second lenses, each lens having a different field of view. The lenses may be compound lenses made up of multiple lens elements aligned on the same optical axis. In preferred embodiments, the lenses have apertures less than {fraction (1/10)}xe2x80x3 in diameter. The lenses themselves may include inexpensive molded plastic spherical lens elements. Each lens may include two concave-convex lenses with spherical convex surfaces and diameters less than xc2xdxe2x80x3. The lens elements may be snap fit in an alignment tube or barrel to maintain the lenses in position on a common optical axis in back-to-back relationship.
Advantageously, a moving optical element is provided for selectively providing an image to the image sensor through one of the first or the second lenses. The moving optical element may be a fast acting, moving mirror which folds the optical path of at least one of the lens units. An electronic servo mechanism may be used to control the mirror.
Imaging engine embodiments of the present invention may also include a target illumination source which emits light from a forward face of the engine. The illumination source may have two or more illumination fields or beams corresponding to different fields of view or focal depths of the lens assembly. The illumination source may produce a relatively broad beam of lower output intensity to illuminate a target barcode relatively close to the reader, and a relatively narrower beam of higher output intensity to illuminate a target barcode relatively far from the reader. A portion of the illumination source for producing the lower output intensity illumination beam may comprise one or more light emitting elements with a wide divergence pattern located relatively near a principal optical axis of the lens assembly. Additional elements may be used to provide a longer range illumination beam of higher output intensity. In a preferred embodiment, this function is performed by light emitting elements with a narrow divergence pattern located relatively farther from a principal optical axis of the scanner. These latter elements may be plural light emitting diodes each associated with a focusing lenslet located in a front face of the reader.
In an illumination system using plural light emitting elements, certain intensity variations may occur across the illuminated field. In preferred embodiments of the present invention, image processing circuits and/or software compensates signals from the image sensor for known variations in illumination provided by the illumination source.
Preferred embodiments may include circuitry and/or software for processing and decoding image data received from the image sensor. An image sensor produces electronic signals corresponding to a two-dimensional array of pixel information for a target image. This data is analyzed by circuitry/software based systems to determine black and white threshold information. The pixel data is divided into subimages, for example, 32xc3x9732 pixel subimages. These subimages are analyzed for properties known to be associated with various types of optical codes and known to distinguish a particular code from other codes and from environmental (non-code) images.
In preferred embodiments, an auto-discrimination system determines which subimages contain codes of a particular type and the coordinates in the pixel data array of certain boundaries or features of preliminarily identified code areas. Further circuitry and/or software extracts one or more waveforms from the data in image areas of interest. Peaks and valleys in the waveforms are identified. A digitizer converts this information to electronic signals corresponding to the code content of the image. The output of the digitizer is applied to a decoder which provides a decoded data output or indicates a failure to decode, which may then trigger additional auto-discrimination in a further attempt to identify and locate a decodable image.
The image processing circuitry/software may be employed to analyze a portion of the projected aiming pattern in the pixel information produced by the image sensor. This information may be used to provide feedback to the processing circuitry to help identify an image area in which a target barcode is located. Such image processing may also be used for determining the distance between the reader and the target barcode. This information, in turn, may be used to select an illumination level for a subsequent image to be decoded, and to control the illumination source. Alternatively, the information may be used to select a focal distance for a subsequent image to be decoded and to control the lens assembly associated with the image sensor to switch different objective lenses into the input optical path of the image sensor.
The scope of the present invention is as defined in the claims.